Paleoecology of Late Paleozoic Pteridosperms from Tropical Euramerica^ William A
Total Page：16
File Type：pdf, Size：1020Kb
Journal of the Toirey Botanical Society 133(1), 2006, pp. 83—118 Paleoecology of Late Paleozoic pteridosperms from tropical Euramerica^ William A. DiMichele^ ^ Department of Paleobiology, NMNH Smithsonian Institution, Wasliington, DC 20560 Tom L. Phillips Department of Plant Biology, University of Illinois, Urbana, IL 61801 Hermann W. Pfefferkorn Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19106 DiMlCHELE, W. A. (Department of Paleobiology, NMNH, Smithsonian Institution, Washington, DC 20560), T. L. PHILLIPS (Department of Plant Biology, University of Illinois, Urbana, IL 61801), AND H. W. PFEFFERKORN (Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19106). Paleo- ecology of Late Paleozoic pteridosperms from tropical Euramerica. J. Torrey Bot. Soc. 133: 83—118. 2006.— Late Paleozoic pteridosperms are a paraphyletic group of seed plants that were prominent elements of tropical ecosystems, primarily those of wetlands or the wetter portions of seasonally dry environments. Because the group is more a tradition-based, historical construct than a well circumscribed phylogenetic lineage, the wide variance in ecological roles and ecomorphological attributes should not be surprising. Pteridosperms can be the dominant canopy trees in local habitats, prominent understory trees, scrambling ground cover, thicket-formers, or liana-like plants and vines. Some species appear to have been weedy opportunists, although this ecological strategy seems to be a minor part of the wide spectrum of pteridosperm life habits. Most pteridosperms appear to have preferred wetter parts of the landscape, though not standing water, and relatively nutrient-rich settings (in comparison with groups such as tree ferns or lycopsids). Of the Paleozoic pteridosperms as traditionally circumscribed, only the peltasperms survived to become major elements in the Mesozoic. However, these plants may have been part of a derived seed-plant clade that also includes the corystosperms and cycads (see Hilton and Bateman, this volume), indicating that only the most derived of the Paleozoic pteridosperm lineages, those that appear to have evolved initially in extrabasinal settings, persisted into the Mesozoic. Key words: Carboniferous, Devonian, paleoecology. Paleozoic, Permian, pteridosperms, seed ferns. The pteridosperms ("seed ferns") are an easy cortical regions. Many are vines or appear to group to caricature but difficult to characterize. have climbing plants at the root of their local Phylogenetically diverse, and denizens of a wide branch of the phylogenetic tree. And large to range of terra firma habitats, the pteridosperms super-sized seeds seem to have been prominent generally have been circumscribed by the pos- parts of the pteridosperm equation in some of session of "fern-like", compound fronds while the better known groups. Yet, there are clear ex- also being the bearers of seeds. All are woody, ceptions to all of these attributes that make the but most not prominently so, the wood often ap- group quite difficult to delineate ecologically. pearing to be an evolutionary afterthought rather Just as there is no typical pteridosperm mor- than the principal support tissue, except in a few phology (Galtier 1986), there likewise is no typ- clades. In addition, pteridosperms generally ical pteridosperm ecology, even though many of have monoaxial stems and sclerenchymatous the putative clades within the 'pteridosperms' are ecologically rather uniform and narrowly ' The authors wish to acknowledge the following distributed within the broader spectrum of eco- sources of support: The Evolution of Terrestrial Eco- logical resource space. systems Program of the Smithsonian Institution (WD) and the National Science Foundation, Earth Sciences Few Paleozoic pteridosperms persist into the Division, Grant EAR-0207848 (HWP). Mesozoic, with the exception of the peltasperms. - We thank Robert Gastaldo, Jason Hilton, and Dan Of several lineages, the meduUosans and lygi- Chaney for discussion about the ideas presented in this nopterids may be considered the "classic" Pa- manuscript and for sharing expertise and unpublished data. For detailed comments and suggestions on earlier leozoic seed fern groups, both best known from versions of the manuscript we are indebted to Richard the Pennsylvanian, in both adpression and pet- Bateman, Chris Cleal, Jean Galtier, and Hans Kerp. rifaction preservation. Other forms such as Cal- ' Author for correspondence: E-mail: dimichele@si. listophyton, the mariopterids, the peltasperms, edu Received for publication September 2, 2005, and in and most of the latest Devonian and Mississip- revised form October 18, 2005. pian seed-plant taxa are treated traditionally as 83 84 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 133 pteridosperms largely because of their posses- among those groups traditionally identified as sion of some mixture of attributes, such as com- pteridosperms. Nonetheless, groups with shared plex, frond-like leaves, sclerotic cortex, or ra- morphological characteristics can be identified, dially symmetrical seeds. Unfortunately, the de- often with confidence, even though the subse- tailed relationships among these groups are rath- quent relationships among these "pteridosperm" er poorly understood at the level of genera and groups are not well understood in many cases. species, even though a general phylogenetic ar- Phylogenetic pattern is relevant to ecological chitecture for the group does exist (Rothwell and pattern because roles and environmental re- Serbet 1992, 1994; Hilton and Bateman, this sponses may be quite similar within clades and volume). can be quite different from one clade to another In this paper we review what is known or sus- Clade membership seems to specify the basic pected of the ecology of the Paleozoic pterido- ecological attributes of many families, genera sperms, focusing on the ecologies of the earliest and species. This is quite clear in Paleozoic eco- forms, the autecologies of the broad diversity of systems, especially those of the pre-Permian Pennsylvanian-aged groups, the place of pteri- wetlands (DiMichele and Phillips 1996a) where dosperms in the synecology of Pennsylvanian species richness is low and several Linnean tropical wetlands, and finally the ecological pat- class-level groups, each characterized by a dif- terns inferred for pteridosperms in the ensuing ferent basic body plan, dominate different eco- Permian seasonally dry tropical lowlands. The logical portions of landscapes. Such patterns general message is that pteridosperms were di- have been identified even in modern systems verse and ecologically heterogeneous, with a (Prinzing et al. 2001). For the late Paleozoic, the wide range of complex growth morphologies root of this extreme clade-by-environment eco- and roles in ecosystems of the time. logical partitioning lies in the Middle to Late A note on geological terminology used in this Devonian evolutionary radiation of basic plant paper: In order to date the events or fossil oc- body plans (DiMichele et al. 2001). The result- currences discussed herein, we will use the clas- ing pattern persisted until the Pennsylvanian, sical European Carboniferous stage names that when global environmental changes began to are most appropriate for terrestrial deposits. disrupt the established dominance hierarchy, These names, from youngest to oldest, are: Tour- leading to the rise of dominance by a variety of naisian, Visean, Namurian, Westphalian, and seed-plant groups throughout the world (Gastal- Stephanian. We will refer to the Tournaisian as do et al. 1996). "early" Mississippian and the Visean as "mid- In the case of the pteridosperms, certain pat- dle" Mississippian. The old, informal substages terns of clade-by-ecology congruence are evi- of the Namurian (A, B, C) also will be used, dent. Examples include the general liana-like where Namurian A (= Serpukhovian) is latest habit of the members of the Pennsylvanian- Mississippian (Early Carboniferous) and the oth- Permian Mariopteridaceae (Krings et al. 2003b) er two stages represent the earliest Pennsylva- and Pennsylvanian Lyginopteridaceae (Phillips nian (Late Carboniferous). The use of the more 1981, Speck 1994, Gastaldo et al. 2004), the numerous, formal substages of the Namurian scrambling to liana-like habit of the members of would be cumbersome and imply a precision the Mississippian Calamopityaceae (Galtier that is neither possible nor necessary in this re- 1992), the tree architectures of the woody lygi- view. We use Asselian, Sakmarian, Artinskian, nopteroids of the Mississippian (Galtier 1992), and Kungurian for the Early Permian, and Guad- and the gracile habit and opportunistic life his- alupian for the earliest stage of the Middle tory of the earliest seed plants (Rothwell and Permian. Scheckler 1988, Scheckler 1986b). Another in- stance is the vine/climber ancestry of the Med- Pteridosperm Phylogeny: Relevance to ullosales (Dunn et al. 2003b), which manifests Ecology. The pteridosperms are a paraphyletic itself in the persistent polystelic anatomy of the group as resolved by most phylogenetic analyses group. Such anatomy can be found both in spe- (Rothwell and Serbet 1994, Nixon et al. 1994, cies with weak-stemmed, leaning, thicket-form- Doyle 1996, Hilton and Bateman, this volume). ing habit and in those forms with upright, mono- However, because these phylogenetic analyses podial tree habit (Pfefferkorn et al. 1984, Wnuk focus mainly on a